An image forming apparatus such as an electrophotographic laser beam printer, copying machine, multi-functional peripheral equipment, and the like uses a powder color material or liquid color material (to be referred to as toner or ink) to form a visible image on a printing medium.
For example, an electrophotographic image forming apparatus performs laser scanning on the surface of a charged image carrier and exposes it in accordance with image data, thereby forming an electrostatic latent image. The apparatus then supplies color materials to the image carrier to develop the electrostatic latent image, and transfers and fixes the developed image on a printing medium to form a visible image on it.
A printing machine other than the electrophotographic one also forms a desired visible image by overlaying a plurality of colors on a printing medium on the basis of image data.
Image data for forming a visible image is obtained by converting a document image into digital data by an image scanning apparatus. The image data is input to the image forming apparatus or printing machine through an image processing apparatus (work station or personal computer) or apparatus having an image processing function which is connected to the image forming apparatus or printing machine. Image data for forming a visible image may be generated by a work station or personal computer.
[Generation of Colors]
Most printing machines or image forming apparatuses perform a printing process by using four-color image data of cyan (C), magenta (M), yellow (Y), and black (K). These four colors are used because they have high affinities for forming an image since color reproduction on a printing medium is based on a subtractive process. In addition, the colors of the color materials (ink or toner) are generally these four colors. That is, color information of image data for forming a visible image and that of image data processed by the work station, personal computer, or apparatus having the image processing function described above are often expressed by these four colors.
Recently, the image forming apparatus has achieved various improvements in order to meet a demand for a high-quality image represented by an image having silver halide photo quality. A printing machine sometimes uses spot colors such as fluorescent colors, and the spot colors have also been used in image forming apparatuses other than the printing machine.
For example, in order to improve tone, spot color materials are used which are obtained by making CMYK lighter and called light color materials. Alternatively, spot colors called dark color materials can be used. In another example, a color material having a hue other than CMYK is used as a spot color material to widen the gamut of formable images and achieve a high-quality image. In this manner, the image forming apparatus forms a high-quality visible image using the spot colors.
However, color information about spot colors may not exist in image data for forming a visible image in some cases. More specifically, image data for forming a visible image has CMYK color information and the like, but may not have information about spot colors.
Image data for forming a visible image is generally page description language (PDL) format data (to be referred to as “PDL data” hereinafter). When an image forming apparatus forms a visible image, PDL data is input to the image forming apparatus. Mostly PDL data has CMYK color information and the like in the same manner as text data, graphic data, etc., but has no color information about spot colors. Therefore, in general, an image processing apparatus included in an image forming apparatus separates CMYK information into CMYK colors and spot colors.
FIGS. 1 to 3 are graphs for explaining generation of colors for forming an image.
When the colors are generated from color information from the input PDL data (that is, when CMYK information is input to generate CMYK colors), as illustrated by a curve 101 in FIG. 1, the density of the corresponding color can be increased in accordance with an increase in the density of the color information of the input PDL data. FIG. 1 shows the relationship between the input and output densities for one color. However, colors other than the colors included in the color information of the input PDL data are to be generated, as illustrated in FIGS. 2 and 3, for example, the densities of two or more colors are determined on the basis of the color information of one color. For example, a curve 101′ in FIG. 2 represents density reproduction by a light color material having low density, and a curve 102′ represents density reproduction by a dark color material having high density. Examples of light and dark colors of the same color system are light cyan and dark cyan, and light magenta and dark magenta. Printing using the light color material 101′ is performed in an input density range between 0 to a neighborhood of an input density 104, and printing using the dark color material 102′ is performed in an input density range of the input density 104 and more. Accordingly, in FIG. 2, an input density range, where the light color material and the dark color material are mixed, is very narrow or is not existence.
FIG. 3 is a graph showing density reproduction by dark and light color materials of another type. A curve 101″ in FIG. 3 represents density reproduction by the dark color material, and a curve 102″ represents density reproduction by the light color material of the same color system as the dark color material. As the input density increases from 0, printing using the light color material 102″ is performed, and printing using the dark color material 101″ is started from an input density 103. In FIG. 3, the input density range, where the light color material and the dark color material are mixed, is broader than that of the graph shown in FIG. 2.
[Trapping Process]
Many methods for processing image data have been employed in obtaining high quality image, e.g., PDL data, for forming a visible image, or processing intermediate data obtained by converting the image data. One example is known as a trapping technique. A trapping technique suppresses reductions in the quality of a visible image caused by misregistration of a color which, in turn, occurs due to an imperfect arrangement of the color, imperfect visible image formation, imperfect transportation of a recording medium, or the like. For example, Japanese Patent Laid-Open No. 2000-165694 describes details of this technique.
FIGS. 4 to 8 are views which help explain a trapping process.
An image 1 shown in FIG. 4 includes regions 2 and 3 formed by different colors. When image data representing the image 1 is input, the regions of the image 1 are extracted and a new region (trap region 4 shown in FIG. 5) is generated near the boundary of the two regions 2 and 3. Color information is generated so that the trap region 4 shares the color of the two regions 2 and 3.
FIG. 6 shows a case wherein no trapping process is performed. When misregistration of the color of a region 12 occurs, the background color of a printing medium shows itself (a so-called “blank portion” appears) between the regions 12 and 8. This causes a reduction in the quality of the visible image. FIG. 7 shows a case wherein a trapping process is performed. Even when misregistration of the color of a region 12 occurs, no blank portion readily appears owing to a trap region 13 and a reduction in the quality of the visible image can be prevented.
This trapping process is performed on image data having CMYK color information such as PDL data and intermediate data obtained by converting the PDL data, and it prevents the reduction in quality of a visible image caused by misregistration.
[Trapping Process and Color Generation]
Color information of image data to be processed by a trapping process is generally expressed using CMYK. However, the image forming apparatus sometimes forms a visible image using colors including spot colors other than CMYK. For this reason, when generating colors from trapping-processed image data having CMYK color information, a color contrary to the purpose of the trapping process may be generated.
For example, a change in the color information of cyan (C) by the trapping process may not be reflected in the density of the color of cyan (C). In this case, the effect sought through use of the trapping process cannot be achieved sufficiently. For example, assume that a trapping process is performed on the image data of the image 1 shown in FIG. 4 and a trap region sharing the color (e.g., cyan (C)) of regions 2 and 3 is generated. After this trapping process, for example, the color (C) of the region 2 is separated, the value of the color (C) becomes 0, and the color (C) is separated into colors green (G) and magenta (M) serving as spot colors. In this case, the trap region and region 3 share the same color (C), but the trap region and region 2 do not share the same color (C), therefore the density sought through the use of the trapping process is not reflected in the color. As a result, as shown in FIG. 8, a trap region 5 does not share the color of a region 6 which is the spot color corresponding to the region 2. When misregistration of the color forming the region 6 occurs in this state, a blank portion appears.